1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer or a copier. More particularly, the present invention relates to an alignment of a sheet with an image of an intermediate-transfer-type image forming apparatus which starts an image forming operation before a paper feed operation.
2. Description of the Related Art
In a conventional image forming method for an apparatus such as a color laser printer or a color copier, images which are formed by toner of each of four colors (Y: yellow, M: magenta, C: cyan, and K: black) are sequentially formed on a single photosensitive member as an image carrier. The images carried by the photosensitive member are sequentially transferred to a transfer member such as an intermediate transfer member to be superposed, and then transferred to a sheet. A disadvantage of this method is that a considerable amount of time is required in forming the full color image on the sheet.
In recent years, a color laser printer having a plurality of photosensitive members serving as image carriers has been developed to meet the demand for higher image-forming speed. In such a color laser printer, an optical apparatus scans independently scans a surface of each photosensitive member with each of a plurality of light beams to form an image in each CMYK color. The color images are superposed on an intermediate transfer belt and finally transferred onto a sheet to form a color image. This is called an intermediate transfer system (hereinafter referred to as tandem type). A tandem type laser printer performs image forming with a plurality of colors (4 colors) using a plurality of photosensitive members at the same time. Accordingly, the time for forming the color image on a sheet can be significantly reduced compared to the aforementioned method.
Next, a specific configuration and movement of the tandem type laser printer will be described referring to FIG. 1. First, laser scanners 11Y, 11M, 11C, and 11K emit a laser beam onto a surface of photosensitive members 13Y, 13M, 13C, and 13K which is charged by charge rollers 15Y, 15M, 15C, and 15K to form an electrostatic latent image. Next, development units 16Y, 16M, 16C, and 16K cause toner to adhere to the electrostatic latent image to visualize the image. The toner adhering to the photosensitive members 13Y, 13M, 13C, and 13K is sequentially superposed on an intermediate transfer belt 17 to form a color toner image.
On the other hand, a sheet 21 in a cassette 22 is fed by a feeding roller 25 at such timing that the sheet 21 matches a toner image on the intermediate transfer belt 17 at a secondary transfer roller 29. Then, the sheet 21 is conveyed by a conveyance roller pair 27 to the secondary transfer roller 29. A full-color toner image on the intermediate transfer belt 17 is transferred to the sheet 21 by the secondary transfer roller 29. The full-color toner image is fixed to the sheet 21 by a fixing unit 30 and a full-color printed matter is produced.
However, when the sheet 21 is aligned with the toner image, the sheet 21 is occasionally double-fed to a position where separation rollers 26a and 26b are arranged due to friction or static electricity, etc. between the top sheet and the next sheet in the cassette 22. In this case, the distance from the sheet 21 to the secondary transfer roller 29 where the image is transferred becomes short, and occasionally, the sheet 21 reaches the transfer position on the secondary transfer roller 29 earlier than the image on the intermediate transfer belt 17 is conveyed to the secondary transfer roller 29. Further, in some cases, a slip of the feeding roller 25 delays the sheet 21, which reaches the transfer position at the secondary transfer roller 29 later than the image on the intermediate transfer belt 17 is conveyed to the secondary transfer roller 29. These cases cause misalignment of the toner image and the sheet 21.
In order to solve this problem, Japanese Patent Application Laid-Open No. 11-249525, for example, discusses a method for aligning a sheet with a toner image regardless of a double feeding of the sheet in a cassette or a slip of the feeding roller.
FIG. 10 illustrates an example of a conventional technique in which conveying of the sheet 21 is temporarily stopped to align with the toner image. A thick line in FIG. 10 is a plot of a Y image which is formed on a photosensitive drum disposed at the most upstream side of the intermediate transfer belt 17. The Y image is a yellow primary image that is first transferred to the intermediate transfer belt 17. Areas shaded with vertical lines show that the sheet 21 is fed at timing earlier than desired timing. More specifically, these areas show a positional change of the sheet 21 when double feeding of the sheet 21 occurs in the cassette 22. On the other hand, areas shaded with horizontal lines show that the sheet 21 is fed at timing later than desired timing. More specifically, these areas show a positional change of the sheet 21 when a slip of the feeding roller 25 occurs in the cassette 22.
Generally, in the tandem type using the intermediate transfer belt 17, the image forming is started before the paper feed operation from the cassette 22. When printing is instructed, the image forming on each photosensitive member is started in an order from the Y image whose photosensitive member is disposed at the most upstream side of the intermediate transfer belt 17, the M image, the C image, to the K image. Then each image formed on the corresponding photosensitive member is transferred onto the intermediate transfer belt 17 in the order of Y, M, C, and K. In the meantime, the sheet 21 is fed from the cassette 22 at timing earlier than when the toner image is formed on the intermediate transfer belt 17.
The sheet 21 fed from the cassette 22 is conveyed by the conveyance roller pair 27. When the sensor 28 detects the sheet 21, the conveyance of the sheet 21 is stopped. A conveyance time of the sheet 21 from starting the paper feed from the cassette 22 until detecting the sheet 21 by the sensor 28 is measured in advance. A stop time of the conveyance of the sheet 21 is calculated according to the conveyance time. The conveyance of the sheet 21 is restarted after the calculated stop time. In this way, the sheet 21 is aligned with the toner image on the intermediate transfer belt 17, or in other words, the leading edge of the sheet 21 is aligned with the top of the toner image.
According to a configuration of the image forming apparatus, a stepping motor 45, which facilitates control of position and speed in an open loop control system, is used for driving the feeding roller 25 and the conveyance roller pair 27. The stepping motor 45 has a characteristic that it tends to step out if it is restarted before vibration generated by stoppage is not sufficiently reduced. Thus, the stepping motor 45 requires a relatively long stop time until the vibration is sufficiently reduced.
Thus, the conveyance of the sheet 21 needs to be stopped until the vibration of the stepping motor 45 is sufficiently reduced even in the case where the feeding roller 25 whose stop time is the shortest, slips. If the sheet 21 is stopped for a longer time, the paper feed interval increases and throughput is reduced. In order not to reduce the number of sheets printed per unit time, processing speed needs to be increased. This can be achieved by increasing a speed of the stepping motor or by adjusting image forming conditions, which may, however, increase the cost or complicate a control system.
In these days, the sheet 21 is aligned with the toner image on the intermediate transfer belt 17 without stopping the sheet 21. This technique is, for example, discussed in Japanese Patent Application Laid-Open No. 2004-333609. FIG. 11 illustrates an example of a conventional technique in which the sheet 21 is continuously conveyed to the position where it is aligned with the toner image. As described referring to FIG. 10, the thick line is a plot of the Y image which is formed on the photosensitive drum disposed at the most upstream side of the intermediate transfer belt 17. The areas shaded with vertical lines show that the sheet 21 is fed at timing earlier than desired timing. More specifically, these areas show a position of the sheet 21 when double feeding of the sheet 21 occurs in the cassette 22. Further, the areas shaded with horizontal lines show that the sheet 21 is fed at timing later than desired timing. More specifically, these areas show a position of the sheet 21 when a slip of the feeding roller 25 occurs in the cassette 22.
When printing is instructed, the image forming on each photosensitive member is started in an order from the Y image whose photosensitive member is disposed at the most upstream side of the intermediate transfer belt 17, the M image, the C image, to the K image. Then each image formed on the corresponding photosensitive member is transferred onto the intermediate transfer belt 17 in the order of Y, M, C, and K. In the meantime, the sheet 21 is fed from the cassette 22 at timing that is ideal for the alignment of the leading edge of the sheet 21 with the top of the toner image. The sheet 21 is conveyed by the conveyance roller pair 27 and detected by the sensor 28.
A conveyance time of the sheet 21 from starting the paper feed from the cassette 22 until detecting the sheet 21 by the sensor 28 is measured in advance. According to the conveyance time, the conveyance speed of the sheet 21 is accelerated or decelerated until the sheet 21 is conveyed to a predetermined control end point at the upstream of the secondary transfer roller 29 where the sheet 21 is aligned with the toner image. When the sheet 21 is conveyed to the control end point, the conveyance speed is reset to a predetermined constant speed. This technique eliminates the need for temporarily stopping the stepping motor 45 which drives the feeding roller 25 and the conveyance roller pair 27 and thus contributes to reducing the paper feed interval and preventing decrease of throughput. In addition, this technique can improve printing efficiency without increasing the processing speed.
Further, the image forming apparatus may set a plurality of processing speeds according to a type of the sheet 21. It is known that a fixing capability of the sheet 21 depends on its thickness, material, and surface smoothness. For example, a thick sheet tends to absorb more heat from the fixing unit 30 than a plain sheet or a sheet which is thinner than the thick sheet. Thus, in order to melt the toner sufficiently and firmly fix the image on a thick sheet, the thick sheet needs to pass through the fixing unit 30 at a slow speed.
In this case, fixing the image can be enhanced by conveying the sheet 21 with slower speed while it passes through the fixing unit 30. This technique is discussed in Japanese Patent Application Laid-Open No. 6-208262. However, according to the configuration of the image forming apparatus, the sheet 21 may be conveyed not only by the fixing unit 30 but also by the secondary transfer roller 29 or the conveyance roller pair 27. Accordingly, a speed (conveyance speed of sheet) of the whole process of the image forming apparatus including the image forming unit but excluding the fixing unit needs to be decreased.
Japanese Patent Application Laid-Open No. 11-249525 discusses a technique for detecting a delay in the paper feed and accelerating the conveyance speed in order to make up for the time delayed. Further, Japanese Patent Application Laid-Open No. 2004-333609 discusses a technique for correcting a variation of a pick-up time of a feed unit, by which a sheet can be fed without stopping the conveyance of the sheet.
However, the conventional image forming apparatuses described above have the following problems.
In a case a plurality of processing speeds are used according to a type of the sheet, the conveyance speed of the sheet 21 is accelerated or decelerated so that the sheet 21 is aligned with the toner image without stopping the conveyance of the sheet 21. In such a case, a speed adjustment range (a setting range of the drive frequency) of the stepping motor 45 which drives the feeding roller 25 and the conveyance roller pair 27 increases.
For example, if a processing speed of a thick sheet 21 is ¼ of a processing speed for a plain sheet (normal processing speed), the drive frequency of the stepping motor 45 driving the feeding roller 25 and the conveyance roller pair 27 is 1000 pulse per second (pps) for the plain sheet and 250 pps for the thick sheet 21, and furthermore, if a change rate of the conveyance speed necessary in aligning the sheet 21 with the toner image is ±20%, the total drive frequency of the stepping motor 45 which drives the feeding roller 25 and the conveyance roller pair 27 will range from 200 to 1200 pps, which is considerably wide.
Generally, the stepping motor 45 tends to generate vibration and noise at a lower speed drive and step out due to low torque at a higher speed. If a vibration absorber or a larger stepping motor 45 is used to cope with the above-mentioned characteristics, the cost of the image forming apparatus will be increased.